Smoke detectors, gas detectors and like apparatus for signalling an environmental alarm condition are commonly connected to a remote central power supply, usually a low voltage direct current source. Only a two-wire line is needed to supply the DC power to a detector unit for detection although additional wires may be used for other purposes. It is possible not only to transmit power through the two-wire line but also to transmit signals from the remote power source to one or more detector units by reversing the polarity on the two power supply wires. It is one object of the present invention to provide apparatus which utilizes polarity reversal in two-wire supply lines and a single alarm indicator.